3.1. Field Pea and Oat Yields and Yield Components
In all three years, the plants of field pea were significantly (
p < 0.01) taller in the second mowing phase compared to the first mowing phase (
Table 4). These differences were recorded equally for single-grown field pea and its mixtures with oats in all three years of research. For the second and the third year, plants were significantly higher in the single-grown field pea compared to its mixtures. Interactions were recorded in the second year between crop and nitrogen (
p < 0.01), and phase, crop and nitrogen (
p < 0.05).
The higher number of internodes was measured in the second mowing phase compared to the first for all three years; however, these differences were not significant in the third year. The significantly higher number of internodes in the second mowing phase, for both mixtures and sole crops, was measured in the first year only. In the first and the third year (2016, 2018), the higher number of internodes of field pea was measured in the mixture compared to sole field pea. There were no significant interactions for this trait.
In three years of testing, a significant influence of all three factors on the biomass yield was measured for the field pea and oats intercrops (
p < 0.01). Interaction between the mowing phase, sowing norm, and nitrogen was found for all three years of testing (
p < 0.05,
p < 0.01,
p < 0.05, respectively). Analysis has also shown first-order interactions between different sowing norms and nitrogen in the second (
p < 0.01) and the third year (
p < 0.05), as well as phase and nitrogen interaction in the second year (
p < 0.01) (
Table 4). Significantly higher yields of biomass were achieved in the first mowing phase compared to the second mowing phase. In all three years, this is equally measured for both species and their mixtures. Overall the lowest biomass yields were measured in single-grown field pea. The highest biomass yields were measured in the 100:30% mixture for all three years and oats in the first and second experimental years. Mixture 100:15% achieved the highest yields along with the 100:30% mixture and sole oats only in the first year (average 19.94 t ha
−1). Nitrogen fertilizer had a significant effect on all crops. On average, the highest yields were achieved with 80 kg ha
−1 N (21.28 t ha
−1), followed by 40 kg ha
−1 N (19.35 t ha
−1), and the lowest without nitrogen fertilizer (17.75 t ha
−1). This trend of biomass yield increase from 0 to 80 kg N ha
−1 has been observed for all three years and all mixtures and sole crops independently, with only one exception of field pea in the third year. On average, nitrogen had the highest impact on the yield of oats, where the difference between control and the fertilized crop is 5.15 t ha
−1 of raw mass. For field peas, these differences are 2.02 t ha
−1, for mixtures 100:15%—3.29 t ha
−1 and mixture 100:30%—3.67 t ha
−1.
Following a similar trend to biomass yields, hay yields were significantly affected by all factors, except for the mowing phase in the first year. No significant interactions have been observed in the first year. In the second year, first-order interactions have been recorded between all tested factors (phase × crop, phase × nitrogen, crop × nitrogen) with high significance (
p < 0.01), as well as second-order interaction between all tested factors (
p < 0.05). In the third experimental year, first-order interactions have been recorded between the mowing phase and nitrogen (
p < 0.05), crop and nitrogen (
p < 0.05), as well as second-order interactions between all tested factors (
p < 0.01). Mixtures and sole crops had varying yields depending on the time of their harvest and the year. Regarding the crop factor, the highest yields have been recorded for sole oats in the first and the third year (average 4.99 t ha
−1) and mixture 100:30% in the second and the third year (average 4.82 t ha
−1), followed by a mixture 100:15% (average 4.44 t ha
−1), and sole field pea (average 3.17 t ha
−1). Nitrogen fertilizer increases the yields of hay for mixtures as well as sole crops (
Figure 2). However, some differences within the particular treatments are notably present in the third year (
Figure 3). It is important to note that the third year had the highest amount of natural nitrogen in the soil and the best soil quality overall. Consequently, the nitrogen fertilizer in the third year most certainly had a diminishing effect as compared to the second year, which had lower soil quality and nitrogen content. On average, the highest yields were achieved with 80 kg ha
−1 N (4.99 t ha
−1), lower with 40 kg ha
−1 N (4.27 t ha
−1), and the lowest with the control treatment, 0 kg ha
−1 N (3.82 t ha
−1).
A significant effect of all tested factors was observed for crude protein yields. First-order interactions were recorded between phase and mixture (
p < 0.01). First and second-order interactions were recorded for all tested factors in the second and the third year (
p < 0.01). In the first and the second experimental year, crude protein yields were higher in the first phase compared to the second, while in the third year, yields were higher in the second mowing phase. Values for three years of testing show the highest protein yields for a sowing norm of 100:15% (average 730.4 kg ha
−1), followed by the 100:30% mixture (692.7 kg ha
−1), field peas (633.5 kg ha
−1), while the lowest yields of crude proteins were obtained from pure oats (456.1 kg ha
−1). On average, the mixture 100:15% achieved higher protein yields compared to 100:30% in all three years. These differences were not as significant in the second and the third year when we observed only averages. However, when the nitrogen is excluded, mixture 100:15% achieves significantly higher protein yields compared to the 100:30% mixture in all three years (
Figure 4,
Figure 5 and
Figure 6). Only once the nitrogen is applied do the protein yields of the 100:30% mixture achieve similar yields to the 100:15% mixture. Regardless, across mixtures and sole crops, the best results are achieved with the addition of nitrogen compared to the control treatments, 80 kg ha
−1 N—average 704.1 kg ha
−1, followed by 40 kg ha
−1 N—average 637.6 kg ha
−1 N, and 0 kg ha
−1 N—average 542.9 kg ha
−1. In terms of the mowing phase, the yields were highly dependent on the particular mixture or sole crop and the year of testing.
3.2. Field Peas Share in Yield and Land Equivalent Ratio
Significant effects on the field peas’ share of the yield was recorded for all tested factors. The mowing time had significant effects in the second and the third year (p < 0.01). Share in yield was significantly different between mixtures in all three years (p < 0.01), while the nitrogen had a significant effect in the first (p < 0.01) and second experimental years (p < 0.05). Significant interactions (p < 0.05) were recorded in the first year between the mowing phase and nitrogen (p < 0.05), and in the third year between the mowing phase and crop (p < 0.05).
Although the differences between the first and the second mowing phase were significant regarding the share in the yield of field peas, these differences were not consistent on a year-to-year basis. The sowing norm had a significant influence on the share of field pea in the yield (
p < 0.01), and the higher share was recorded in the 100:15% mixture for all three years (
Table 5). Almost equal yields are observed between field pea and oats in the mixtures, even though field pea has a significantly higher sowing percentage. The average share in the yield for the mixture 100:30% is 50.25%, and for the mixture 100:15%, 60.62%. Nitrogen had a significant effect on the share of field peas in the yield as well, and it tends to decrease as the nitrogen increases. For all three years of testing, the amount of 80 kg ha
−1 N decreased the share of field peas by 8.26% and the amount of 40 kg ha
−1 N by 4.42%. Regarding the particular mixture and nitrogen levels, for the 100:30% mixture, the difference is mainly observed between no nitrogen application and applied nitrogen, regardless of the applied level, since there were no differences between 40 and 80 kg ha
−1 N. For the mixture 100:15%, the lowest field peas share in yield was recorded when 80 kg ha
−1 N was applied; however, differences compared to other levels were not significant throughout the years.
Significant effects of the tested factor on the land productivity were observed for the mowing phase in the first year and nitrogen in the second and the third year. No significant differences were observed between the mixtures. Second-order interactions were observed in the second year (p < 0.05) and first-order interactions between phase and nitrogen in the third year (p < 0.01). The productivity of intercropped field peas and oats is higher compared to sole crops. Intercropped field peas and oats almost always utilize the land more productively compared to sole crops. The average LER value within three factors and three years of testing is 1.18. The LER was significantly higher in the second mowing phase compared to the first, only in the first experimental year.
3.3. Economic Contributions of Field Pea and Oats Mixtures
In
Table 6, the inputs and output results for field pea and oat mixtures, fertilized by different nitrogen levels, are given. For both mixtures and each treatment, three-year average costs were calculated. The analysis included machinery operation cost with the labor force, raw material costs, output, net return, and benefit–cost ratio. It can be noted that total costs are primarily dominated by raw material costs (63%), 64% for a 100:15% and 100:30% mixture. With a share of as much as 50%, the most significant component of raw material costs is seed costs. However, the costs of machine operations, which include labor costs, account for about 36% of total costs. The economic analysis of the production for both mixtures indicates that a small net profit is realized even without the application of nitrogen fertilizer. For treatments without nitrogen, net returns of EUR 28.4 and EUR 39.4 were found in a mixture of 100:15% and 100:30%, respectively. The net returns were improved by the use of nitrogen fertilizer but with differences between mixtures and fertilization levels. While the net returns are increased by 123.6%, the application of 40 kg ha
−1 N for 100:15% mixture burdens the total costs by 4.6%. A significant increase in the net returns was found in this mixture, especially by applying 80 kg ha
−1 N. Here, crop fertilization burdens the total production costs by 6.9%, as the net returns are increased by 164.4%. However, it was found that with the increase in oats in the sowing norm, the economic efficiency of the nitrogen decreases. Namely, the costs of applying nitrogen fertilizer to the mixture 100:30% are 4.5% and 6.7% of the total production costs, while the profit increases by 3.6% and 137.3% by applying 40 and 80 kg ha
−1 N.
Although the benefit–cost ratio of any treatment is not greater than 1, it can be observed that on the values of the nitrogen-free treatment are the lowest. In the mixture of 100:15%, with the application of 40 kg ha−1 N, the benefit–cost ratio was increased by 100%, while the treatment with 80 kg ha−1 N increased by 133.3%. However, in the mixture of 100:30%, 40 kg ha−1 N did not increase the Benefit–cost Ratio, while the treatment of 80 kg ha−1 N showed an increase of 112.5%.